problem while using Chip select pin along with a GPIO pin

While using Chip Select 1, we have observed that we are not able to assert PSC6_3 (configured as GPIO) while chip select is kept asserted. That is, even if we extend wait period of Chip Select 1 and assert PSC6_3 in the next instruction itself, actual assertion occurs only after chip select gets de-asserted.In our application we needed to assert PSC6_3 while keeping Chip Select asserted. Could you please tell us why this unexpected delay is occuring?